Plate glass is usually a soda-lime glass. The batch is melted and refined as described above, great care being taken to remove all the "gall," which is skimmed off immediately before the casting. An especially strong pot is used, which will stand the strain of lift ing from the furnace while full of melted glass. The furnace is constructed with brick-lined, cast iron doors, which open to permit the removal of the glass. The melting and annealing furnaces are often joined, so that the latter may be heated with waste heat. Some times several plates are annealed at one time.
The chief uses of plate glass are for windows and mirrors. A considerable quantity of rough plate, unground as it comes from the annealing furnace, is used for flooring and sky-lights.
Window glass is always blown. It is usually a soda-lime glass, and the batch is melted and refined in the usual manner, either in pots or in tanks. After refining, the glass is allowed to become pasty and then the blower begins his work. His chief tool is the "pipe," a straight piece of iron tubing four or five feet long, usually provided with a mouthpiece. He dips the pipe into the soft glass, which is called "metal," and gathers a lump on the end. Then, by blowing through the pipe, while whirling it between the palms of his hands he forms a hollow globe of glass. This is re-heated in a special furnace called a "Glory-hole," (Fig. 57), until soft, rolled on a flat surface, and then swung in a Vertical. circle, with occasional blowing through the pipe until the globe has elongated into a hollow cylinder, closed at one end and opening into the pipe at the other. In order to have plenty of room for the vertical swinging, the workman stands on a ,plank or bridge placed across a rather deep pit. The -closed end of the cylinder is reheated until soft, and -then blown- out ; the small opening thus made is en larged by means of the widening tongs. The pipe is detached by touching its point of attachment with a wet stick, and the edges of the still soft glass are trimmed with shears. A hollow cylinder open at both ends is thus formed, and is cut lengthwise with a diamond. It is then put into the flattening furnace, in such a position that the cut is on the upper side. The heat being sufficient to soften the glass, the 'Cylinder slowly opens and spreads out on the floor of the furnace in a flat sheet. It is then transferred to the annealing furnace for blown ware (Fig. 58).
This consists of a long oven, heated at one end and cool at the other. A system of endless iron bands carries the glass from the hot to the cool end of the oven. Sometimes the glass tube to be annealed is placed on a large horizontal table, usually built of slabs of stone, and carefully balanced so as to revolve easily and slowly by means of a gear, while the seg ment passes through a narrow opening in the side of the flattening furnace, where it is exposed to the high temperature. The glass is thus slowly carried out of the furnace into a cooler compartment from which it is removed when nearly cold. The table is chiefly used for window glass.
The glass sheets are now cut into marketable size without any polishing. Since the surface of blown glass is fused and not polished, it is brilliant and hard. Consequently it is less easily scratched or etched and is more durable than plate glass when exposed to the weather.
blowing is an exceedingly fatiguing -labor, And only men of strong constitution and good lungs can do it The mass of glass which a good workman will handle at- one time, averages about 18 pounds, and from it he will form a cylinder over a yard long and a foot in diameter.
Pressed glass is made by the use of a die or mould ; these moulds are quite expensive, but owing to the great number of pieces of the same form and design that are made with slight labor, pressed ware is fairly cheap.
"Tough" or "tempered" glass is produced by a special method of annealing, the articles so treated being capable of withstanding blows and sudden changes of temperature. This tempering is done by plunging the article, while still so hot as to be some what soft, into a bath of oil heated to 100 deg, to 300 deg. C. This sudden "quenching" hardens the surface of the glass but causes internal stresses. If scratched or cut slightly, toughened glass is very apt to fly to pieces, sometimes with great violence. And even after standing a long time, spontaneous fractures often occur. It is mainly used for lamp chimneys.
A process for making hardened glass plates and window lights is employed in which cold metallic sur faces are applied to the glass plates while the latter are still plastic. The sudden chilling imparts an ex ceedingly hard surface to the glass, so that it can be used in exposed situations, as in street lamps.